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Oops, we may have misunderstood our satellite data

March 5th, 2007 by seok · 2 Comments

It seems that the International Satellite Cloud Climatology Project (ISCCP) has given us some “bad” data regarding long-term cloud level studies. Evan et al. argues for this in a letter published in GRL vol. 34, 2007. The abstract reads,

The International Satellite Cloud Climatology Project (ISCCP) multi-decadal record of cloudiness exhibits a well-known global decrease in cloud amounts. This downward trend has recently been used to suggest widespread increases in surface solar heating, decreases in planetary albedo, and deficiencies in global climate models. Here we show that trends observed in the ISCCP data are satellite viewing geometry artifacts and are not related to physical changes in the atmosphere. Our results suggest that in its current form, the ISCCP data may not be appropriate for certain long-term global studies, especially those focused on trends.

After some mind boggling justification, the authors tried to ease those that may be of concern. They wrote that it may be possible that new methodologies could be developed to process the ISCCP data and account for artificial variations caused by viewing geometry in cloudiness. One way would be to simply ignore the data with the artifacts. Another method would be to regress out those “bad” data. The best way, the researchers claim, would be at the data processing stage, because the regression method could not completely account for the effect of changing viewing geometry on the cloud data.

The authors conclude,

… Results from earlier studies based on these trends may be influenced by these non-physical artifacts, and we therefore suggest that development of a correction for the data is warrented. As the number of publications on the subject of climate change continues to grow [...], this paper highlights the need to critically explore the source of any trends in global, multi-decadal satellite data sets.

Studies like these shows us the that how we interpret data is important. In this case, it demonstrates that mis-interpreting data can cause large amounts of uncertainty in our knowledge of cloud cover.

One thing about this article that bothers me is that Evan et al. never measured the magnitude of the error caused by the viewing geometry artifact. This is of concern for me, because I fear that this study may provide some grounds for global warming skeptics. But then, I do not think we can explain global warming from levels of cloudiness, so I may not have anything to be worried about.

Tags: instruments · satellite

2 responses so far ↓

  • 1 seand // Mar 6, 2007 at 3:47 pm

    Another recent study of trends in cloudiness was using ground-based met observers. …The paper is

    Warren, S. G., R. M. Eastman, and C. J. Hahn (2007), A Survey of Changes in Cloud Cover and Cloud Types over Land from Surface Observations, 1971-1996, Journal of Climate, 20, 717-738.

    Bottom line: They detect no change in global cloudiness, although some changes in cirrus. This somewhat at odds with the old ISCCP studies, but I guess its in broad agreement with this most recent ISCCP paper.

  • 2 Leif G Liland // May 11, 2007 at 2:10 pm

    Why is global cloud amount so interesting?

    On CO2-science they comment this survey by Warren, S. G., R. M. Eastman, and C. J. Hahn (2007)

    “More specifically, they say that “the time series of total-cloud-cover anomalies for individual continents show a large decrease for South America, small decreases for Eurasia and Africa, and no trend for North America.” They also state that “the zonal average trends of total cloud cover are positive in the Arctic winter and spring, 60°-80°N, but negative in all seasons at most other latitudes”

    So, the total ends up close to zero, but doesn’t all of these decreases and increase contribute to global warming?
    Increased cloud cover in the Arctic winter will increase the temperature, so will a reduced cover “in all seasons at most other latitudes”.
    Remember most of the global warming after 1980 is a “high latitude warming”.

    I know CO2 science is probably not the most balanced on this subject but I assume their citation is correct.

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